The present invention relates to a gas sensor installed in an exhaust system of an internal combustion engine for an automotive vehicle to control the air-fuel ratio of fuel mixture supplied into a combination chamber of the engine.
FIG. 12 shows a conventional gas sensor 9 installed in an exhaust system of an automotive engine to control the air-fuel ratio of fuel mixture. The gas sensor 9 includes a sensor element 1002 inserted in a cylindrical housing 1010, a measured gas cover 1011 provided at a distal end side of the housing 1010 to cover a distal end side of the sensor element 1002, and an atmospheric air cover 1012 provided at a proximal end side of the housing 1010 to cover a proximal end side of the sensor element 1002.
According to this conventional gas sensor 9, the inside space of the measured gas cover 1011 is a measured gas environment 1110, while the inside space of the atmospheric air cover 1012 is an atmospheric air environment 1120. The clearance between the sensor element 1002 and the housing 1010 should be gastightly sealed, to separate or isolate these environments 1110 and 1120 from each other. Furthermore, the clearance between the sensor element 1002 and the housing 1010 should be watertightly sealed. Watertightly sealing the clearance between the sensor element 1002 and the housing 1010 brings the effect of preventing fuel liquid from entering from the measured gas environment 1110 during the engine startup operation and also brings the effect of preventing condensate from entering from the measured gas environment 1110 during the engine stopped condition.
Regarding a seal arrangement between the sensor element 1002 and the housing 1010, the U.S. Pat. No. 6,510,728 discloses a method of using inorganic powders or a molded product of inorganic powders. More specifically, as shown in FIG. 12, the inorganic powders are filled into the clearance between the sensor element 1002 and the housing 1010. And then, the inorganic powders are pressed to form a powder filler 1091. Subsequently, an insulator 1192 and a metallic ring 1193 are disposed on the powder filler 1091. Alternatively, it is possible to manufacture a temporary molded article beforehand by temporarily molding inorganic powders into a shape substantially identical with a space formed between the sensor element 1002 and the housing 1010. The temporary molded article is disposed in this space, and then pressed to fill and provide a gastight and watertight sealing between the sensor element 1002 and the housing 1010.
The measured gas cover 1011 consists of an outer cover 1111 and an inner cover 1112. The atmospheric air cover 1012 consists of a main cover member 1121 and an outside cover member 1122. Furthermore, an atmospheric air side insulator 1013 holds lead wires 1015 of the sensor element 1002. The lead wires 1015 are inserted in an elastic insulating member 1016 provided at the proximal end side of the atmospheric air side insulator 1013.
In general, there is the tendency that recent advanced engines discharge high-temperature exhaust gases. It is therefore required that a seal arrangement between the sensor element and the housing possesses excellent high-temperature durability. According to a conventional sealing method, an appropriate amount of additive agent is mixed with inorganic powders to increase the density of pressed inorganic powders and enhance the sealing properties. However, conventionally known additive agents tend to decompose at high temperatures. Therefore, the conventional known additive agents cannot be used for the gas sensors which are subjected to high-temperature environments.
If a gas sensor uses inorganic powder filler containing no additive agents, sealing properties (e.g. gastightness and watertightness) of this sensor will soon deteriorate in accordance with cumulative time of engine operations even if good sealing properties are assured in a brand-new condition of this sensor. To enhance the sealing properties, it may be possible to use a higher pressure in the process of pressing inorganic powders so as to increase the density of the inorganic powder filler. However, applying a higher pressure to inorganic powders may lead to generation of cracks in the sensor element because the sensor element is a fragile ceramic product which is weak against shock.
Furthermore, as disclosed in the U.S. Pat. No. 5,846,391, it is conventionally proposed to use a powder filler arrangement consisting of three layers of steatite, boron nitride, and steatite to increase the sealing properties. However, a hot press forming operation performed at high temperatures of approximately 2000° C. is required to form the boron nitride layer, although steatite layers can be formed through the sintering operations performed at low temperatures of approximately 500° C. In other words, to realize the hot press forming operation, this prior art technique requires a special furnace having sufficient high-temperature durability. This will increase the thermal energy cost.
In view of the foregoing, it is required to provide an excellent seal arrangement between the sensor element and the housing of a gas sensor, which is capable of assuring excellent sealing properties and accordingly capable of maintaining long-lasting watertightness and gastightness, and requires no special apparatus in the manufacturing processes and accordingly realizes easy manufacturing method.
Meanwhile, there is a conventional gas sensor including an element assembly, consisting of a sensor element and a surrounding cylindrical insulating tube, which is inserted in a cylindrical housing. A measured gas cover is provided at a distal end side of the housing to cover a distal end side of the sensor element. And, an atmospheric air cover is provided at a proximal end side of the housing to cover a proximal end side of the sensor element. The gas sensor having such an arrangement is not free from the above-described problems, as well. Thus, it is required to provide an excellent seal arrangement assuring long-lasting sealing properties (i.e. watertightness and gastightness) between the element assembly and the housing of a gas sensor, which requires no special apparatus in the manufacturing processes and accordingly realizes easy manufacturing method.